linux-ext4 - Re: [PATCH v1 01/15] mm: add zone device coherent type memory support

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Date:   Thu, 12 May 2022 13:45:16 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     jgg@...dia.com, david@...hat.com, Felix.Kuehling@....com,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, willy@...radead.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v1 01/15] mm: add zone device coherent type memory support


On 5/11/2022 9:58 PM, Alistair Popple wrote:
> Alex Sierra <alex.sierra@....com> writes:
>
> [...]
>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index fedb82371efe..d57102cd4b43 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1995,7 +1995,8 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags)
>>   					TTU_SYNC)))
>>   		return;
>>
>> -	if (folio_is_zone_device(folio) && !folio_is_device_private(folio))
>> +	if (folio_is_zone_device(folio) &&
>> +	    (!folio_is_device_private(folio) && !folio_is_device_coherent(folio)))
>>   		return;
>>
>>   	/*
> I vaguely recall commenting on this previously, or at least intending
> to. In try_to_migrate_one() we have this:
>
> 		if (folio_is_zone_device(folio)) {
> 			unsigned long pfn = folio_pfn(folio);
> 			swp_entry_t entry;
> 			pte_t swp_pte;
>
> 			/*
> 			 * Store the pfn of the page in a special migration
> 			 * pte. do_swap_page() will wait until the migration
> 			 * pte is removed and then restart fault handling.
> 			 */
> 			entry = pte_to_swp_entry(pteval);
> 			if (is_writable_device_private_entry(entry))
> 				entry = make_writable_migration_entry(pfn);
> 			else
> 				entry = make_readable_migration_entry(pfn);
> 			swp_pte = swp_entry_to_pte(entry);
>
> The check in try_to_migrate() guarantees that if folio_is_zone_device()
> is true this must be a DEVICE_PRIVATE page and it treats it as such by
> assuming there is a special device private swap entry there.
>
> Relying on that assumption seems bad, and I have no idea why I didn't
> just use is_device_private_page() originally but I think the fix is just
> to change this to:
>
> 		if (folio_is_device_private(folio))

Thanks Alistair. This worked fine. I'll drop patch 4 and update this 
patch in the next series version.

Regards,
Alex Sierra

>
> And let DEVICE_COHERENT pages fall through to normal page processing.
>
>   - Alistair